This invention relates to gas turbine engine propulsion systems and, more particularly, propulsion systems of the flight maneuverable variety.
The high velocity imparted to exhaust gases of a gas turbine engine by the exhaust nozzle provides thrust for propulsion. This thrust is substantially opposite to the direction of the flow of exhaust gases exiting the nozzle. Consequently, if the direction of the exhaust gases is changed, the direction of propulsive thrust is correspondingly varied. Typically, aircraft gas turbine engines are provided with nozzles which are fixed in the axial direction, and aircraft maneuvering is accomplished solely by airframe control surfaces. Advanced aircraft configurations contemplate, and may even require, the selective redirection (or vectoring) of gas turbine engine thrust in order to enhance aircraft performance and to provide the aircraft with operational characteristics heretofore deemed impractical. For example, if the exhaust of a conventionally installed gas turbine engine is directed downwardly, rather than rearwardly, to a direction substantially perpendicular to the engine longitudinal axis, the resulting upward thrust would provide direct lift for the aircraft and, if properly controlled, a vertical take-off and landing capability. Similarly, thrust vectoring during flight can greatly increase aircraft maneuverability since the thrust force can augment the maneuvering forces of the aircraft control surfaces such as elevators, ailerons, and rudders. In order to accomplish such thrust vectoring, a device is required to efficiently and practically alter the direction of gas turbine engine exhaust nozzle gases.
Thrust vectoring may be employed in essentially two types of applications. First, it may be used in vertical take-off and landing (VTOL) applications where aircraft operation is at low speed and where continuous vector angle capability up to essentially 90.degree. is required for generating aircraft lift. Secondly, thrust vectoring is employed at relatively high aircraft speeds to achieve combat maneuver capability, the range of vectoring being limited to approximately 30.degree. or 40.degree.. The fundamental difference between these two concepts is that the VTOL application generates system-lift by simply deflecting the engine flow, whereas an in-flight vectoring application utilizing the principal of supercirculation provides lift augmentation that is several times greater than the vertical thrust component of the VTOL application. As is well known in the art, supercirculation refers to the additional wing lift generation due to directing airflow out of, or over, a wing in such a manner as to effectively change the aerodynamic shape of the wing. This lowers the required angle of attack at high subsonic maneuver conditions, thus enabling the aircraft to make high G turns with less drag.
It is predicted that drag reductions in excess of 40% are attainable at typical combat conditions when an aircraft incorporates a flight maneuverable propulsion system for lift augmentation wherein the engine exhaust flow is expelled through the wing in such a manner as to provide the additional lift through supercirculation. This drag reduction allows the engine to be sized significantly smaller than otherwise possible.
However, requirements which have heretofore been associated with the development of such a system have been that:
exhaust system turning losses must be minimized; PA1 most combat aircraft require afterburning (or augmentation) and the burner must be optimized to provide the necessary combustion efficiency in a limited axial length; PA1 the gas turbine engine exhaust stream must be matched to the wing trailing edge to minimize installation drag over the entire flight envelope; PA1 since the afterburner is installed within the wing, cooling must be provided to maintain structural integrity; and PA1 actuators should be kept simple, and their number to a minimum.
The problem facing the gas turbine engine and aircraft designers, therefore, is to provide a flight maneuverable propulsion system which can efficiently and effectively incorporate the aforementioned advantages while meeting these key requirements.